1. Field of the Invention
This invention relates to a method of preparing zinc oxide films and, more particularly, to a method for making zinc oxide films which are utilizable as piezoelectric films.
2. Description of the Prior Art
Zinc oxide films have been used as piezoelectric films for surface acoustic wave filters, vibrators, resonators and the like. For example, zinc oxide films are applied to surface acoustic wave filters in which a zinc oxide film is excited by transducing electrodes and surface acoustic waves are generated in the film. In order to generate desired surface acoustic waves, the film is generally required to have a thickness greater than a half wave length (1/2.lambda.) of the surface acoustic wave to be generated.
So far, such zinc oxide films have been produced by carrying out radio-frequency sputtering with a target of zinc oxide. In such a conventional method, a rate of formation of the film is very low and, for example, of the order of about 1 .mu.m per hour. Thus, when a zinc oxide film with a 25 .mu.m thick is to be produced, it takes about 25 hours. This results in the considerable increase of a manufacturing cost of surface acoustic wave transducers.